The present invention relates generally to vehicle navigation systems. More specifically, the invention relates to methods and apparatus which provide periodic, on-the-fly calibration of readings from a vehicle's odometer sensor to ensure accurate determination of the vehicle's position by the navigation system.
Vehicle navigation systems have traditionally employed a variety of sensors to determine the position, speed, and heading of a vehicle. For example, global positioning system (GPS) sensors have been employed to detect signals from GPS satellites which are, in turn, employed by the navigation system to determine the position of a vehicle. On-board sensors such as magnetic compasses and gyroscopes have been employed to sense the vehicle's heading as well as the angular acceleration accompanying changes in the vehicle heading. For measuring distances traveled by a vehicle, navigation systems have traditionally employed the vehicle's odometer signal. It is evident that the accuracy of the data received from these various sensors is essential to the reliable determination of the vehicle's position.
The accuracy of data received from a vehicle's odometer is influenced by a number of factors. Because an odometer typically detects wheel revolutions as representative of travelled distance, the tire size is directly related to the accuracy of the reported travel distance. For current navigation systems, once the vehicle's tire size is known, it is manually programmed into the navigation system to properly correlate wheel revolutions to travelled distance. However, it is well known that the size of a vehicle's tires change over time as they wear down from contact with the road. Moreover, factors such as the air pressure of the tires and the weight loaded on the vehicle at any given time produce variation in travel distance reported by the odometer. The tire size may be periodically reprogrammed into the system to account for such variations, but this is obviously impractical in that a difficult manual reprogramming would frequently be required, possibly every time the navigation system is used.
Another potential source of error in measured distance reported by an odometer is a mismatch between the odometer's pulse rate and the pulse rate setting of the navigation system. Odometers generate a pulse train in which a specific number of pulses (e.g., 2000) represents a unit distance (e.g., a mile). For example, Nissan vehicles employ a pulse rate of 2000 pulses/mile while Ford vehicles employ a pulse rate of 8000 pulses/mile. Therefore, each navigation system must be configured to correspond to the type of vehicle in which it is installed. Otherwise very large scale errors may result. If, for example, the pulse rate setting in a navigation system installed in a Ford corresponded to the pulse rate of a Nissan, an error factor of four would be introduced. The pulse rate setting is typically done before a navigation system is installed and is difficult to change where, for example, the odometer in the vehicle is changed, or the navigation system is installed in a different vehicle. Thus, while detection of the error may be elementary, correction of the error remains problematic.
It is therefore apparent that there is a need for a convenient technique by which odometer measurements may be rendered reliable and accurate despite the many unpredictable variations encountered over the course of time. There is also a need for a technique to determine whether a navigation system's pulse rate setting corresponds to the pulse rate of the associated odometer, and to reset the pulse rate setting if it is found to be in error.